Method of ultrasonic coupling

ABSTRACT

A method of assuring contact between an ultrasonic transducer and the skin of a subject during an ultrasonic monitoring operation utilizing a gel comprising polyvinylpyrrolidone and propylene glycol.

AU 335 EX- Patent 0 n9. v

Bunnell et al. 14 1 Jan. 16, 1973 [54 METHOD OF ULTRASONIC COUPLING 3,269,173 8/!966 Von Ardenne -1211 2 v 1 1 i 3%??238 i513?! 322221321:31111113111: "31321322 Branchville; Gum", BlOOm- 3,661,146 5/1972 Peronneau 6161 ..128/: v field, both of NJ [73] Assignee: Hothnan-La Roche Inc., Nutley, OTHER PUBLICATIONS NJ, Brydson; Plastics Materials, D. Van Nostrank Co., lnc., 1966, page 286; Sci. Lib., to Group I45. [22] 1971 Ladany et al.; Physiologically Active Solutions for Ap [2]] App]. No.: 207,985 plication to the Skin; Chem. Abstracts," 1961; Vol.

55, 2l497b. Related US. Application Data [62] Division of Ser. N0. 3,509, Jan. 16, 1970, abandoned.

Attorney-Samuel L. Welt et al.

52 I. ..128 2V I28 24 A, 260 33.4 R, 1 U s C l [424/80 [57] ABSTRACT [51] Int. Cl. ..A6lb 10/00 A m thod of assuring ontact between an ultrasonic 0i sflli'ch 2 24 N transducer and the skin of a subject during an ul- R trasonic monitoring operation utilizing a gel comprising polyvinylpyrrolidone and propylene glycol. [56] References Cited 5 Claims, No Drawings UNITED STATES PATENTS 5/1957 Kerwin ..424/ X METHOD OF ULTRASONIC COUPLING This is a division of application Ser. No. 3,509 tiled Jan. 16, 1970, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to the art of biomedical instrumentation, that is, the art of electrically instrumenting a live subject to permit monitoring of selected physiological functions of the subject. More particularly, the present invention relates to an improved method of assuring contact between an ultrasonic transducer and the skin of the subject utilizing an ultrasonic energy coupling gel which is especially adapted for use as a coupling medium between a subject's skin and a transducer of an ultrasonic generating device.

The main function of an ultrasonic coupling gel is to assure that a proper contact is maintained between the transducer of an ultrasonic biomedical monitoring device and the skin of the subject. There are several known methods of assuring contact between a device such as herein described and the skin of the subject. One such method involves placing the pickup device on the skin and spraying it with an adhesive substance to hold it firmly in place. This method has the pronounced disadvantages of drying out almost immediately and losing function permanently if the contact is broken with the skin. The most desirable of the known methods of contact assurance is the coupling paste or gel. Such a paste or gel is particularly desirable in terms of comfort to the patient in the non-surgical situation as it allows for some movement by the patient without causing the unpleasant pulling of the skin and discomfort felt when a contact assurance medium drys out. A gel may also allow for slight movements of the sensing device itself without the loss of a sensing contact if the gel is sufficiently conductive to afford accurate measurements. It is usually possible to utilize a coupling gel with almost any ultrasonic recording device which employs a pickup instrument in contact with the skin.

In order to be fully effective in the clinical or hospital situation, an ultrasonic coupling gel must successfully meet a number of rigorous requirements. To meet these requirements, a gel must possess all of the following characteristics:

A. An ultrasonic velocity equal to that of body tissue. This characteristic allows for complete freedom from ultrasonic beam deflection, thus allowing the shape of the beam to be controlled solely by the transducer design;

B. A very low incident of attenuation of ultrasonic energy. A high incidence of attenuation would result in a lowered signal to noise ratio thus making it more difficult to detect the return signals electronically;

C. An acoustic impedance approximately equal to that of human body tissue. This is important as when there is an acoustical impedance mismatch, a portion of the ultrasonic signal is lost by being reflected at the interface between gel and skin;

D. Extremely good wetting characteristics so as to thoroughly wet both the transducer surface and the skin. A poor wetting situation results in a poor transfer of ultrasonic energy;

E. The capability to remain exposed in constant use for extended periods of time without drying out;

F. The capability to remain sticky and tacky over extended periods of time so that contact will be maintained if the transducer or other sensing device is inadvertently slightly pulled away from the skin;

G. Freedom from causing allergenic reaction and also freedom from irritation to the skin even after contact for extended periods of time;

B. Non-obnoxiousness to the patient, e.g., a gel must be non-staining, pleasing in appearance and water soluble to afford easy removal from the skin by washing;

l. Thermal stability in the normal clinical situation, e.g., there should be no decrease in viscosity or tackiness of the gel upon exposure to temperatures in the range which may be produced by the human body; and

J. The capability to simultaneously conduct ultrasound transmissions and insulate against electrical energy. This is essential so the danger of the subject receiving an electric shock by contact with the ultrasonic transducer can be maintained at an absolute minimum.

The above criteria for an ultrasonic coupling gel are set forth in detail to emphasize the exact, rigid physical/chemical property requirements which a composition must meet in order to be an effective ultrasonic coupling media. The compositions of the present invention have, surprisingly, been found to possess all of the aforementioned properties thus completely satisfying the requirements for an ultrasonic coupling media as set forth above.

In contrast to the compositions of the present invention, prior art coupling gels or pastes have two main disadvantages, in addition to being comparatively less sensitive, e.g., viscosity changes and rapid drying out. Most of the presently available commercial preparations, although packaged in a solid form, have been found to become liquid, or at best semi-solid, upon exposure to body temperatures. This property is extremely deterious to the overall effectiveness of the product as the liquified gel will flow from between the transducer and the skin resulting in poor or no ultrasonic coupling. A disadvantage often inherent in the aforementioned spray-on electrode adhesives is that they dry out in a relatively short period of time, thus causing loss of signal;

When any ultrasonic coupling media eventually dries out, there should ideally be no residue film which may eventually be difficult to remove rapidly by washing. It is, therefore, desirable that both the coupling gel and any residue which it might leave upon drying must be water soluble and thus easily removable by washing. The coupling gel of the present invention also meets this criteria.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, ultrasonic coupling gels are provided which comprise polyvinylpyrrolidone and propyleneglycol. The amount of polyvinylpyrrolidone in the gels according to the present invention may vary widely. Generally, however, a concentration of from about 2.0 percent by weight to about 50.0 percent by weight polyvinylpyrrolidone based on the weight of the composition is contemplated herein. More preferred are gels containing from about 3.5 percent to about 25.0 percent by weight polyvinylpyrrolidone. Especially. preferred are compositions comprising from about 6.85 percent to about 13.70 percent polyvinylpyrrolidone. The remainder of the gel is substantially all propylene-glycol.

In addition to the two basic components, the ultrasonic coupling gels of the present invention may contain minor amounts of other ingredients such as, for example, perfumes and preservatives. The only requirement for the utilization of these additives is that they do not interfere in any way with the function of the gel. Especially preferred among the preservative substances known in the art is a combination of methylparaben and propylparaben. It is preferred to utilize each of these preservatives in concentration of about 0.1 percent by weight based on the total weight of the preparation.

In the preparation of the gels of the present invention, it is preferred to utilize high molecular weight polyvinylpyrrolidone, e.g., commercially available products having a molecular weight of about 350,000. The percent of polyvinylpyrrolidone in the gel will vary inversely with the molecular weight of the product utilized. Generally, where polyvinyl pyrrolidone having a molecular weight of 350,000 is utilized, a gel of the proper consistency and tackiness is obtained utilizing from about 3.5 percent to about 25.0 percent polyvinylpyrrolidone. The determination of the present concentrations of polyvinylpyrrolidone of other molecular weights in the gel to achieve the desired characteristics is considered to be within the purview of one skilled in the art.

The preparation of the ultrasonic coupling gels of the present invention consists of dissolving the preservatives, perfumes and the like in propylene glycol and thereafter adding the desired amount of polyvinylpyrrolidone with heating to about 100 C. and stirring. It is essential to the efficient utilization of the coupling gels of the present invention that they be free of air bubbles which may act to distort the sound waves. Accordingly, the gel may be made free of air bubbles which may act to distort the sound waves. Accordingly, the gel may be made free of air bubbles by several methods recognized in the art. A preferred method is to hold the finished preparation at about 100 C. without stirring for from about 1 to about 4 hours, usually about 2 hours until most of the air bubbles have dissipated and then allow it to cool under reduced pressure. After the air bubbles have been eliminated from the gel, it may then be packaged in suitable, air-tight containers.

As has been stated, the ultrasonic coupling gels of the present invention fully meet the rigorous criteria set forth above to assure contact between an ultrasonic transducer and the skin of the subject. In addition, the gels of the present invention possess exceptional stability of vital characteristics. By this is meant the gels of the present invention may be utilized in the taking of ultrasonic readings over extended periods of time, e.g., up to 24 hours and beyond, without drying out or losing other important characteristics such as, for example, tackiness, ultrasonic conductivity, and the like. In addition, the gels of the present invention are cosmetically pleasing and possess good shelf life stability.

It was surprising and unexpected that a gel comprising polyvinylpyrrolidone and propylene glycol was found to meet the aforementioned rigorous charac teristic requirements as extensive experimentation has shown that preparations consisting of similar substances known in the art such as, for example, polyvinyl alcohol, in various vehicles, failed to meet these criteria in one or more particulars. It is theorized that gels comprising polyvinylpyrrolidone and propylene glycol meet the varlety of rigorous requirements for an ultrasonic coupling gel while combinations of other similar substances do not because of hydrogen bonding which occurs during formation of the gel of the present invention. This theory is supported by measurements of infrared absorption based on the carbonyl stretching frequency by which readings were made to determine the amount of polyvinylpyrrolidone in the gels of the present invention. These measurements taken on numerous samples of the gels of the present invention repeatedly showed low concentrations of polyvinylpyrrolidone compared to the amounts actually present. It is theorized that these results indicate hydrogen bonding in the gel. It is further theorized that it is this phenomenon which makes the novel gels of the present invention clearly superior to preparations made from other similar substances known in the art.

Example 1 1.0 Gram of methyl paraben, 1.0 g. of propyl paraben and 2.0 g. of perfume base were dissolved in 859.0 g. of propylene glycol. To this solution was added 137.0 g. polyvinylpyrrolidone having a molecular weight of about 350,000 with stirring. The mixture was heated to about C. with continued stirring until solution was completed. The solution was held at about 100C. for 2 hours thus dissipating most of the air bubbles. The gel solution was then placed under a mild vacuum and allowed to cool to room temperature. The completed gel was then packaged in suitable containers.

Example 2 Example 1 was repeated utilizing 68.5 g. of polyvinylpyrrolidone having a molecular weight of about 350,000 and 927.5 g. of propylene glYcol.

We claim:

1. A method of assuring contact between an ultrasonic transducer and the skin of a subject during an ultrasonic monitoring operation which comprises intimately dispersing between and in contact with the skin and said transducer an ultrasonic coupling gel comprising polyvinylpyrrolidone and propylene glycol, said polyvinylpyrrolidone comprising from about 2.0 percent to about 50.0 percent by weight of said gel.

2. The method in accordance with claim 1 wherein said polyvinylpyrrolidone comprises from about 3.5 percent to about 25.0 percent by weight of said gel.

3. The method in accordance with claim 2 wherein said polyvinylpyrrolidone comprises from about6.85 percent to about 13.70 percent by weight of said gel.

4. The method in accordance with claim 3 wherein said gel contains a preserving agent.

5. The method in accordance with claim 4 wherein said preserving agent consists essentially of 0.1 percent by weight methyl paraben and 0.1 percent by weight propyl paraben. 

2. The method in accordance with claim 1 wherein said polyvinylpyrrolidone comprises from about 3.5 percent to about 25.0 percent by weight of said gel.
 3. The method in accordance with claim 2 wherein said polyvinylpyrrolidone comprises from about6.85 percent to about 13.70 percent by weight of said gel.
 4. The method in accordance with claim 3 wherein said gel contains a preserving agent.
 5. The method in accordance with claim 4 wherein said preserving agent consists essentially of 0.1 percent by weight methyl paraben and 0.1 percent by weight propyl paraben. 